1. Field of the Invention
The present invention relates to a color cathode ray tube which improves deflection aberration i.e., distortion of a beam spot, produced by a deflection magnetic field generated by a deflection unit, thereby improving focusing characteristics, and the deflection unit.
2. Description of the Related Art
In general, as shown in FIG. 1, a color cathode ray tube has an envelope 3 comprising a panel 1 and a funnel 2. A phosphor screen 5 comprising of three color phosphor layers for emitting blue, green, and red light rays is formed on the inner surface of the panel 1, and a shadow mask 4 is arranged to oppose the phosphor screen 5. An electron gun assembly 7 for emitting three electron beams B, G, and R is arranged in a neck 6 of the funnel 2. The three electron beams B, G, and R are horizontally and vertically deflected by a defection unit 9 mounted outside a boundary portion between a conical portion 8 and the neck 6 of the funnel 2, thereby scanning the phosphor screen 5. As a result, a color image is displayed on the phosphor screen 5.
As shown in FIG. 2, the deflection unit 9 has a pair of horizontal deflection coils 10 for horizontally deflecting the three electron beams and a pair of vertical deflection coils 11 for vertically deflecting them.
In order to correctly display an image on the phosphor screen 5 in the color cathode ray tube having the above arrangement, the three electron beams B, G, and R must be correctly converged on all of the phosphor screen 5. For this purpose, a self-convergence in-line type color cathode ray tube is generally adopted. This color cathode ray tube generally uses, as the electron gun assembly 7, an in-line type electron gun assembly emitting three electron beams arranged in line, the center beam G and the pair of side beams B and R emitted from the electron guns passing through the same plane. In the color cathode ray tube provided with this in-line type electron gun assembly, specific non-uniform magnetic fields used as deflection magnetic fields are formed by the defection unit 9, thereby converging the three electron beams B, G, and R on all of the phosphor screen 5. In general, as the un-uniform deflection magnetic field generated in the self-convergence in-line type color cathode ray tube, a pincushion type magnetic field is used as a horizontal deflection magnetic field, and a barrel type deflection magnetic field is used as a vertical magnetic field. By using the above magnetic fields, the three electron beams B, G, and R arranged in line passing through the same horizontal plane ca be converged at one point on the phosphor screen 5.
When the magnetic field is generated in this manner in the in-line type color cathode ray tube, coma aberration, in which convergence between the center beam G and the side beams B and R is shifted in a peripheral portion of the screen, may be produced.
In order to correct this coma aberration, in techniques disclosed in Published Examined Japanese Patent Application Nos. 51-26208 and 54-23208, a magnetic member to be coupled to a magnetic field leaking from a rear side of a deflection unit is arranged in an electron gun assembly. In addition, in a technique disclosed in Published Examined Utility Model No. 57-45748, an auxiliary coil is arranged at the electron gun assembly side of the deflection unit and a current in synchronism with a deflection current flowing through a vertical deflection coil is supplied to the auxiliary coil. This generates an intense pin-cushion type magnetic field without using a magnetic member to be coupled to a magnetic field leaking from a rear portion of the deflection unit.
In these conventional color cathode ray tubes, however, a spot of an electron beam on the phosphor screen is still distorted in accordance with deflection. That is, as shown in FIG. 3, a spot 13 of an electron beam deflected by an uniform magnetic field is formed into a substantially true circle on the entire surface of a screen 14. As shown in FIG. 4, however, a spot 13 of an electron beam deflected by a non-uniform magnetic field is distorted into a lateral ellipse having the horizontal direction as its major axis at the end of the horizontal axis (X axis) of the screen 14. That is, as shown in FIG. 5A, the electron beams B, G, and R are distorted by a pin-cushion type horizontal deflection magnetic field 15 such that an upper half of each beam is pushed downward and its lower half is pushed upward by a Lorentz force. At the end of the vertical axis (Y axis) on the screen 14, as shown in FIG. 5B, each of the electron beams B, G, and R is distorted into a lateral ellipse having the horizontal direction as its major axis by a barrel type vertical deflection magnetic field 16 such that a right half of each electron beam is pushed to the right and its left half is pushed to the left by a Lorentz force. The magnitudes of forces applied to the right and left sides of each of the pair of side beams B and R are different from each other, and the direction of a force applied to the electron beam B at the left side of the screen is opposite to that of a force applied to the electron beam R at the right side thereof. Therefore, spots of the side beams B and R at the end of the vertical axis are inclined to cross each other as indicated by reference numerals 13B and 13R in FIG. 4. As a result, focusing characteristics at the peripheral portion of the screen 14 are significantly degraded by deformation or inclination of the beam spots caused by the horizontal or vertical deflection magnetic field 15 or 16. In addition, this degradation in focusing characteristics does not allow high performance of the electron gun assembly.
For this reason, in order to improve the focusing characteristics at the peripheral portion of the screen 14, compromising design must be made in consideration of uniformity of focusing at the central and peripheral portions of the screen 14 at the sacrifice of focusing at the central portion of the screen 14.
Since the auxiliary coil used in Published Examined Utility Model Application No. 57-45748 uses a current synchronized with a deflection current flowing through the vertical deflection coil, the following problem is posed. That is, when an electron beam is to be deflected in the vertical direction, the electron beam is excessively deflected in the vertical direction at the electron gun assembly side of the deflection unit by a magnetic field generated in the horizontal direction on the horizontal axis, and tends to collide against the inner wall of the neck of the funnel. As a result, a portion called a neck shadow which does not emit light rays, because no electron beam reaches there, tends to be formed on the screen. In addition, this auxiliary coil is manufactured by winding a coil around a magnetic member, and a current is flowed through the coil. Therefore, this auxiliary coil is expensive as a correction element, and it is difficult to decrease its manufacturing cost. Furthermore, the deflection unit is often used by changing its impedance in accordance with the type of a receiver of each set maker, and a current to be flowed through the deflection coil is changed in accordance with the changed impedance. Therefore, in order to allow the auxiliary coil to properly operate with respect to the deflection unit, the specification of the auxiliary coil must be changed in accordance with the impedance of the deflection coil, resulting in poor mass-productivity.